4.3 Hydrostatic Weighing and Air Displacement Plethysmography

Body composition analysis is crucial for assessing health and fitness. Hydrostatic weighing and air displacement plethysmography are two methods that estimate body fat percentage by measuring body density. These techniques offer accurate insights into fat mass and fat-free mass.

Both methods rely on similar principles but differ in execution. Hydrostatic weighing submerges individuals underwater, while air displacement uses a sealed chamber. Understanding these techniques helps grasp the complexities of body composition assessment and its importance in nutrition and health.

Hydrostatic Weighing for Body Composition

Principles and Calculations

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• Hydrostatic weighing, also known as hydrodensitometry or underwater weighing, estimates body composition based on Archimedes' principle of buoyancy
  • Archimedes' principle states that the buoyant force acting on a submerged object equals the weight of the fluid displaced by the object, allowing for the determination of body density
• Body density is calculated by measuring an individual's weight both on land and while completely submerged in water
  • The difference in these weights, along with the water density, is used to calculate body density using the following formula: $\text{Body Density} = \frac{\text{Weight in Air}}{\text{Weight in Air} - \text{Weight in Water}} \times \text{Water Density}$
• The relative proportions of fat-free mass (FFM) and fat mass (FM) can be estimated from the overall body density
  • The density of FFM is relatively constant across individuals at approximately 1.1 g/cm³, while the density of FM is lower at around 0.9 g/cm³

Assumptions and Limitations

• Hydrostatic weighing makes several assumptions:
  • The density of FFM is constant at 1.1 g/cm³
  • The density of FM is constant at 0.9 g/cm³
  • Residual lung volume is accurately measured or estimated
• Limitations of hydrostatic weighing include:
  • The need for specialized equipment (hydrostatic weighing tank, underwater scale, helium dilution apparatus for measuring residual lung volume)
  • The requirement for individuals to exhale completely and remain still underwater, which can be challenging for some people (elderly, those with limited mobility or respiratory issues)
  • Potential errors introduced by assumptions about the densities of FFM and FM, particularly in individuals with atypical body composition (athletes, elderly)

Hydrostatic Weighing Procedure

Preparation and Measurements

• Before the procedure, the following measurements are taken:
  • Height and weight using a stadiometer and high-precision scale
  • Residual lung volume using a helium dilution technique or estimated based on age, sex, and height
• The individual wears minimal clothing to reduce air trapped in clothing that could affect the underwater weight measurement

Underwater Weighing Process

• The individual enters a hydrostatic weighing tank filled with water, typically maintained at a temperature of 30-35°C to minimize the effects of water temperature on body density
• The individual sits on a submerged chair or platform connected to a load cell or underwater scale
  • They are instructed to exhale completely and remain still while the underwater weight is recorded
• Multiple underwater weighing trials are conducted, and the highest stable underwater weight is used for calculations
  • The average of the three highest values within 100 grams of each other is typically used

Calculations and Estimations

• Body density is calculated using the individual's dry weight, underwater weight, and the density of the water using the formula mentioned earlier: $\text{Body Density} = \frac{\text{Weight in Air}}{\text{Weight in Air} - \text{Weight in Water}} \times \text{Water Density}$
• The Siri or Brozek equation is then used to estimate body fat percentage from body density:
  • Siri equation: $\text{Body Fat Percentage} = (4.95 \div \text{Body Density} - 4.50) \times 100$
  • Brozek equation: $\text{Body Fat Percentage} = (4.57 \div \text{Body Density} - 4.142) \times 100$

Air Displacement Plethysmography for Body Composition

Principles and Technology

• Air displacement plethysmography (ADP) is a non-invasive method for estimating body composition that measures body density by determining the volume of air displaced by an individual
• ADP systems, such as the BOD POD, consist of two chambers:
  • A test chamber where the individual sits
  • A reference chamber with known volume
• The system measures the pressure changes between the two chambers to determine the volume of air displaced by the individual, which equals their body volume
  • Boyle's law, stating that the product of pressure and volume is constant at a given temperature, is the underlying principle used in ADP

Calculations and Assumptions

• The individual's body density is calculated from their measured body mass and body volume using the following formula: $\text{Body Density} = \frac{\text{Body Mass}}{\text{Body Volume}}$
• The Siri or Brozek equation is then used to estimate body fat percentage from body density, as mentioned in the hydrostatic weighing section
• Assumptions made in ADP include:
  • The density of FFM is constant at 1.1 g/cm³
  • The density of FM is constant at 0.9 g/cm³
  • The individual's thoracic gas volume is accurately measured or estimated
• Thoracic gas volume can be measured during the ADP test using a breathing technique or estimated based on age, sex, and height

Hydrostatic Weighing vs ADP

Similarities

• Both hydrostatic weighing and ADP are densitometric methods that estimate body composition by measuring body density and applying two-compartment models to determine FM and FFM
• Both methods rely on assumptions about the densities of FM and FFM, which may introduce errors in individuals with atypical body composition (athletes, elderly)

Differences in Accuracy and Convenience

• Hydrostatic weighing is considered the "gold standard" for body composition assessment due to its high accuracy and reliability
  • However, ADP has been shown to have comparable accuracy to hydrostatic weighing
• ADP is a more convenient and comfortable method compared to hydrostatic weighing
  • It does not require submersion in water and can accommodate a wider range of individuals, including those with limited mobility
• Hydrostatic weighing requires the individual to exhale completely and remain still underwater, which can be challenging for some people
  • ADP only requires the individual to sit still and breathe normally

Measurement Differences

• Hydrostatic weighing requires the measurement or estimation of residual lung volume, while ADP requires the measurement or estimation of thoracic gas volume
  • Errors in these measurements can affect the accuracy of body composition estimates
• ADP tends to be more expensive than hydrostatic weighing due to the cost of the equipment and the need for trained operators
  • However, the convenience and comfort of ADP may justify the added expense in some settings (research, clinical practice)